Thanks for advice. I've stopped the pineapple and prune juice as I'm aware of the sugar content and giving him very diluted sugarless cranberry juice in the hope I can revert to normal water soon which he wouldn't accept before.
Cranberry has the highest sugar content of any of the juices. Every time you give him something sweet he will hold out for longer and longer waiting for the sweet stuff. They are not stupid and are sugar junkies. How long have you left him without water? In a cool environment they can go 7-10 days. Easily 48 hours with no water and no problems. It is absolutely their nature to hunker down and use less energy/conserve water when not drinking, there is a great article here about thermoregulation that covers this. Some chinchillas - particularly males - drink less than 1 tsp of water per day. Poop size also varies between chins. It may be that he naturally produces smaller poops. Of course he has a lot more energy after you force him to take sweetened water - it has a ton of sugar compared to what is normally in their system. Good luck!
Thanks for information. Didn't realise they could survive on such a small quantity, it's the first time I've had a male after several females. He has only got a choice of water now in bottles or a bowl, I have removed all juice. Although he hasn't touched any he is still very active. As a precaution he is being monitored for 24 hours at my local vets starting tomorrow.